The present invention relates to a patella prosthesis and particularly to the patellar prosthetic surface designed to articulate with the femoral component of a total knee prosthesis. The patellar bearing surface is preferably made of a physiologically inert polymer such as high density polyethylene supported by a physiologically inert metal such as a cobalt-chromium alloy. The patella prosthesis has a plurality of bearing surfaces designed to provide low friction interaction with a metal condylar replacement prosthesis throughout the entire range of leg extension and flexure. The femoral component includes not only a prolonged anterior flange for extended articulation with the patella throughout the excursion of the latter but also includes a medial slot for retention of the cruciate ligaments.
Knee prosthetic devices are now available in many gradations of intrinsic stability depending upon the degree of damage to the knee. The surgeon's goal should be to replace only those functions of the joint which need replacement, preserving as much of the body structure as possible, minimizing the demands placed upon the prosthetic device and preserving as much bone as possible so that the remaining bone can act as foundation for further repair or replacement. The surgical procedure should provide relief from disabling pain, restore functional motion with maximal potential for lasting results and should include adequate provision for satisfactory salvage in the event of failure.
The most common cause of knee trauma or dysfunction is impairment of the frictional interface between the femoral condyles and the cooperating tibial plateau. Such damage is usually brought on by arthritic disease. Where the ligamentous structures of the knee are reasonable intact, a less intrinsically stable resurfacing type prosthesis is indicated than the bulky, complicated so-called "hinge" type prosthesis previously used. The resurfacing type prostheses are well known and are described with slight variations in U.S. Pat Nos. 3,715,763, 3,728,742, 3,774,244 and others. These partial knee replacements usually consist of a combination of a metal component replacing one or both condylar surfaces of the femur and a polymeric replacement for the tibial plateau which may have varying degrees of congruence with the complementing femoral component. The combination is intended to provide a sliding and/or rolling action in cooperation with the existing knee capsule and ligaments thereby providing a functional knee. These mono-condylar or dual-condylar replacements may or may not provide central clearance for the cruciate ligaments which are considered to be important to knee stability and function.
A more intrinsically stable prosthesis is suggested when the capsule and colateral ligaments are intact but the cruciate ligaments are damaged. In this case there is no need to provide for retention of the cruciate ligaments and the means for attaching the prosthesis to the bone can be more substantial, particularly the tibial component which can now incorporate a center spike or shaft for better securement.
When there is poor ligamentous stability, a prosthesis with complete intrinsic stability is more applicable, that is, a device which provides a mechanical linkage between the femoral and tibial components. Many of the presently favored "linked" knee devices use a combination of bearing geometrics and mechanical tee bars in contrast to the more constrained earlier "hinge" devices which offered only one degree of freedom.
It is difficult to place the many knee prosthetic devices into exact categories since there are many variations and combinations in use. In view of the relative newness of these devices, which have been in general use for only a few years, there is still controversy over optimum design. One school of thought holds that devices should approximate the shapes of the natural bone which they replace so that the resultant prosthetic action will more closely approximate the functions and actions of the natural joints. Others reason that since the properties of prosthetic materials are different than those of the natural bones and since material processing and finishing techniques have reproducibility limits, the design must compromise normal anatomical design to achieve lasting functional results within the limitations imposed by the available materials.
Although the resurfacing type prosthesis has only been in general use for the past five years or so, it has been widely accepted. One problem that has been encountered in some cases has been anterior knee pain, patellar in origin, after knee replacement. In some cases, the range of movement post-operatively has been limited to less than 90.degree. because of this patellar pain. The patella serves as a fulcrum for the tendons and ligaments which straighten the leg and disruption of articulation of the patella following knee replacement can increase the quadriceps force required for full extension of the leg by 15 to 30 percent. Pain appears to be aggravated during extension stress such as walking up and down stairs and is concentrated around the patella.
In an effort to alleviate this problem, many of the more recent femoral dual-condylar elements have been designed to provide for prolongation of the anterior flange so that the metal groove articulates with the patella throughout the excursion of the latter. Unfortunately, to provide articulation of the patella and femoral prosthesis throughout the entire range, it was usually necessary to omit the central opening through which the cruciate ligaments pass. Thus, the surgeon was faced with a choice--the important cruciate ligaments could be retained or full patellar articulation could be provided.
The cruciate ligaments stabilize the knee in the anterior-posterior (front-to-back) direction while allowing the joint to flex freely. The two side-by-side ligaments are in the form of an X, one fastened to the anterior of the femur and the posterior of the tibia while the other is fastened posterior to the femur and anterior of the tibia.
The patella is a sesamoid bone imbedded in the extensor tensor of the knee. The primary function of the patella is to increase the efficiency of the quadriceps muscles. The quadriceps femoris is a powerful combination of four muscles which extend or straighten the leg. The patella also serves as a connection between the quadriceps tendon and the patellar ligament. The patella has a ridge on its posterior which slides in the groove between the femoral condyles. The patella in combination with the patellar track and mating portions of the femoral condyles acts as a low friction pulley mechanism for the quadriceps tendon so that the extension force can be carried with efficiency and stability around the curvature of the flexed knee. Unless the patella can slide freely around the knee during flexion and extension, it can cause pain and/or weakness.
Unfortunately, the combination of synthetic or artificial surfaces in contact with natural body materials does not provide a long-term low-friction combination. The patella is meant to function against natural joint surfaces and tends to be unreliable when caused to articulate against metallic surfaces. This problem was solved by a polymeric insert or facing applied to the posterior of a resected patella, which insert would then slide with low friction over the surface of the metal femoral implant.
The problem restated is that known patella prosthetic devices which provide adequate bearing contact with femoral implants through prolonged anterior flanges make no provision for passage of the cruciate ligaments through the femoral component. Femoral implants having a passage for the cruciate ligaments currently offer inadequate bearing contact with the natural patella or with synthetic patella prosthetic surfaces.